PROJECT SUMMARY/ABSTRACT Malformations of the lower genitourinary tract are among the most common birth defects in humans. Hypospadias -- a malformation of the external genitalia characterized by failure of urethral tube closure and incomplete formation of the prepuce (foreskin) and ventral penis -- affects an estimated 1 of every 250 live births. Affected children can have oversized or multiple urethral openings, and those with severe hypospadias are born with ambiguous genitalia. All but the mildest forms require surgical intervention. The etiology of hypospadias is not understood, but recent discoveries of copy number variations (CNVs) in affected individuals suggest a genetic basis for increased susceptibility to hypospadias, which may be compounded by exposure to environmental endocrine disrupting chemicals (EDCs). Our limited knowledge of the most basic molecular mechanisms that pattern the genital tubercle has been an obstacle to understanding how the urethral tube forms during normal development, how hypospadias arises, and how EDCs can affect the gene regulatory networks (GRNs) that orchestrate external genital development. Our group previously showed that deletion of Fibroblast growth factor receptor-2 (Fgfr2) causes hypospadias in mice, and recent studies have illustrated the translational importance of this discovery by identifying deletions affecting the FGFR2 locus in boys with urethral tube defects. In this project, we aim to identify the mechanisms by which Fgfr2 orchestrates urethral tube formation. The goal of Specific Aim 1 is to understand the genetic control of urethra development by dissecting the Fgfr2 GRN to identify downstream targets and to elucidate their functions. The goal of Specific Aim 2 is to understand how Fgfr2 regulates the cellular processes that drive urethral tubulogenesis. These objectives will be accomplished by integrating developmental genetics, cell biology, and novel transgenic mouse models. An immediate translational impact will come through direct comparison of the mouse data to human cases of hypospadias with known CNVs, which will establish the mechanistic links between mutant genotypes and the phenotypes of boys with structural defects of the urethra.